A fuel cell is an electrochemical apparatus wherein chemical energy generated from a combination of a fuel with an oxidant is converted to electric energy in the presence of a catalyst. The fuel is fed to an anode, which has a negative polarity, and the oxidant is fed to a cathode, which, conversely, has a positive polarity. The two electrodes are connected within the fuel cell by an electrolyte to transmit protons from the anode to the cathode. The electrolyte can be an acidic or an alkaline solution, or a solid polymer ion-exchange membrane characterized by a high ionic conductivity. The solid polymer electrolyte is often referred to as a proton exchange membrane (PEM).
In fuel cells employing liquid fuel, such as methanol, and an oxygen-containing oxidant, such as air or pure oxygen, the methanol is oxidized at an anode catalyst layer to produce protons and carbon dioxide. The protons migrate through the PEM from the anode to the cathode. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations:Anode reaction (fuel side): CH3OH+H2O→6H++CO2+6e−  I:Cathode reaction (air side): {fraction (3/2)}O2+6H++6e−→3H2O  II:Net: CH3OH+{fraction (3/2)}O2→2H2O+CO2  III:
The goal in methanol fuel processing is complete methanol oxidation for maximum energy generation shown in the equation. Catalysts that promote the rates of electrochemical reactions, such as oxygen reduction and hydrogen oxidation in a fuel cell are often referred to as electrocatalysts. Electrocatalysts are important because the energy efficiency of any fuel cell is determined, in part, by the overpotentials necessary at the fuel cell's anode and cathode. In the absence of an electrocatalyst, a typical electrode reaction occurs, if at all, only at very high overpotentials. Thus, the oxidation and reduction reactions require catalysts in order to proceed at useful rates.
Carbon Monoxide (CO) Poisoning of the Catalyst
Platinum (Pt), an expensive metal, is the best catalyst for many electrochemical reactions, including methanol oxidation. A major obstacle in the development of methanol fuel cells is the loss of electrochemical activity of even the best electrocatalyst due to “poisoning” by CO. CO is an intermediate in the oxidation of methanol to carbon dioxide (CO2). CO is adsorbed at the surface of the Pt due to its special molecular structure and thus blocks the access of new fuel molecules to the catalytically active Pt centers.
CO is a severe poison to Pt electrocatalysts. It significantly reduces fuel cell performance even at levels of 1-10 ppm. A fuel cell which would be useful for commercial applications would preferably be tolerant of CO levels produced in a relatively uncomplicated fuel system, i.e., 100 ppm or greater.
Substantial effort has been devoted to developing a multi-element catalyst such as Pt—Ru and Pt—Ru—Os. The addition of Ruthenium (Ru), for example, helps convert CO into CO2 and relieves the Pt from being poisoned. Attempts have also been made to further reduce the CO concentration, and particularly through a selective oxidation process for the CO. Conventionally, the oxidation of CO to CO2 occurs in the presence of a catalyst and at temperatures above 150° C.
PEM fuel cells, which have potential application in mass transportation, are very sensitive to CO poisoning. Conventional PEM membranes, such as NAFION™, must contain significant amounts of water to conduct protons from the electrode reactions. Accordingly, PEM fuel cells cannot operate at temperatures over about 100° C., and preferably operate at temperatures around 80° C. At these operating temperatures, CO strongly adsorbs to the Pt catalyst to poison the fuel cell performance.
Thus, there remains a need to reduce the level of CO in the fuel system to improve liquid-type fuel cell performance in an effective and commercially viable manner.
Wettability of the Electrodes
Adequate wetting of the electrodes is another major problem for liquid-type fuel cells. To provide a large reaction area, the electrode structures in a liquid-type fuel cell need to be very porous and the liquid fuel solution needs to wet all pores. In addition, CO2 that is evolved at the fuel side electrode needs to be effectively released from the zone of reaction. Adequate wetting enhances the release of CO2 from the electrode. In PEM fuel cells, the PEM also requires water to be effective in conducting protons.
Conventional gas diffusion type fuel cell anode structures are not suitable for use in liquid-type fuel cells. These conventional electrodes have poor fuel wetting properties. The conventional electrodes, however, can be modified for use in liquid-type fuel cells by coating them with an electrode additive that improve their wetting properties, such as NAFION™ which also serves as a PEM.
U.S. Pat. No. 6,248,460 describes a method of wetting an electrode within a liquid-type fuel cell having a sulfuric acid electrolyte by employing perfluorooctanesulfonic acid as an additive to the fuel mixture of the fuel cell. However, the invention is directed to a very specific application, i.e., improving performance of fuel cells having a sulfuric acid electrolyte, and is not applicable to other types of liquid-type fuel cells, such as fuel cells having a PEM.
“Futile Oxidation” at the Anode
As shown in equation II, the cathode of liquid-type fuel cells is exposed to air where the protons react with oxygen to produce water. Some oxygen will inevitably dissolve in the fuel and will be carried to the anode side of the fuel cell. The oxygen will then be oxidized into oxygen ions by the catalyst on the anode. The oxygen ions will then react with the protons produced on the anode by equation I, and form water on the anode. This “futile oxidation” prevents the transfer of protons from the anode to the cathode and hence diminishes the current generated by the fuel cell reaction. There remains a need to efficiently remove the dissolved oxygen from the fuel of a liquid type fuel cell.
Impurities in the Fuel
Impurities in the fuel of a fuel cell may inhibit the desired electrochemical reaction. For example: a methanol based liquid fuel may contain trace amount of sulphur or metal ions, such as Fe++, Cu++, Cr+, Ni+, and Zn++, that are detrimental to the electrolyte and/or catalyst. The impurities may originate from the fuel supply itself or enter the fuel from elsewhere in the system. Some of the impurities may be chemically adsorbed or physically deposited on the surface of the anode catalyst, blocking the active catalyst sites and preventing these portions of the anode catalyst from inducing the desired electrochemical fuel oxidation reaction.
In the absence of countermeasures, the adsorption or deposition of catalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may, over time, result in a degree of catalyst poisoning which is detrimental to fuel cell performance.
Conventional methods for addressing the problem of fuel impurities include physical filtration and/or chemical treatment of the fuel to remove the impurities. The purification process can be tedious and expensive, and requires specially designed equipment.